Changing shape/size of green collision box
You cannot move or rescale them. From the Blender manual (emphasis added):


  Primitive Shapes
  
  These are best in terms of memory/performance but do not necessarily
  reflect the actual shape of the object. They are calculated based on
  the object’s bounding box. The center of gravity is always in the
  middle for now. Primitive shapes can be shown in the viewport by
  enabling Bounds in the Object ‣ Display panel.


Use a mesh shape to get more precision:


  Mesh-Based Shapes
  
  These are calculated based on the geometry of the object so they are a
  better representation of the object. The center of gravity for these
  shapes is the object origin.


You could, however, use a parent object with a collider if you want more control.